The assembly of a cylinder head and an engine block is normally accomplished by utilizing fasteners, such as bolts or studs having nuts threadably mounted thereon. The fasteners must be preloaded under a predetermined tension to firmly secure the head on the block and to avoid sealing problems in the area of the gasket which is clamped between the head and the block. The preload imposed on the fasteners must also take into consideration any subsequent relaxation or decay of such tension.
A common practice is to employ a torque wrench for preloading the fasteners. One problem occasioned with the use of a torque wrench is that the gauge thereof may not provide accurate readings due to errors induced by factors such as friction at the threads of the driven nut. Even assuming that the preloading procedure is accomplished with good operator control, the tension of the fastener may vary as much as .+-.30%. Although the preload initially imposed on the fasteners by the torque wrench may exhibit substantial error, the subsequent decay of the preload over a prolonged period of service time is normally within acceptable limits, such as .+-.10%.
Another common practice is to preload fasteners of the stud and nut type by first pre-tensioning the stud and then by turning-down the nut by hand or with a driver. Although initial preloading is more accurate than is achieved by use of a torque wrench, the subsequent relaxation or decay of the preload over an extended period of service time may be as high as 50%. This loss in preload primarily occurs because the engaging threads are loaded for the first time when the pre-tensioning load is relieved to thus transfer the load onto the threads of the nut and they are loaded to a stress level beyond their yield point. In addition, burrs and corners of the engaging screw threads are loaded for the first time to further induce creep due to localized high stresses.